Documentation: show screenshots in documentation instead of having links to config itself

[frlan]

Is your feature request related to a problem? Please describe.

When scrolling the documentation and passing by the theme-section there is a list of pre-installed themes. The links don't show any screenshot to give a feeling how it looks, but are links to the config file itself.

Describe the solution you'd like
I'd like to see a preview of the themes.

Describe alternatives you've considered

Additional context